**Core Concept**
Struma ovarii is a rare form of ovarian teratoma that contains thyroid tissue, highlighting the phenomenon of ectopic thyroid tissue. This condition demonstrates the concept of heterotopic tissue, where normal cells or tissues are found in an abnormal location.

**Why the Correct Answer is Right**
Struma ovarii is indeed a type of ovarian teratoma that contains thyroid tissue, often in the form of thyroid follicles. This condition can occur in isolation or in combination with other ovarian teratomas. The presence of thyroid tissue in the ovary is thought to arise from the embryonic migration of thyroid cells to the ovaries during fetal development. The thyroid tissue in struma ovarii can produce thyroid hormones, which may lead to hyperthyroidism in some cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Struma ovarii is not a type of ovarian cyst. While ovarian cysts can be teratomas, struma ovarii specifically refers to a type of teratoma containing thyroid tissue.
**Option B:** Struma ovarii is not a form of ovarian cancer. Although it is a type of ovarian teratoma, struma ovarii is typically benign and does not have a malignant potential.
**Option C:** Struma ovarii is not a term used to describe ectopic thyroid tissue in the thyroid gland. Ectopic thyroid tissue can occur in other locations, such as the mediastinum or the neck, but struma ovarii specifically refers to thyroid tissue in the ovary.

**Clinical Pearl / High-Yield Fact**
Struma ovarii can be challenging to diagnose preoperatively, but the presence of thyroid tissue in the ovary can be identified through histopathological examination of the surgical specimen.

**Correct Answer: D. A rare form of ovarian teratoma that contains thyroid tissue.**